Device including quantum dots

ABSTRACT

A device including an emissive material comprising quantum dots is disclosed. In one embodiment, the device includes a first electrode and a second electrode, a layer comprising quantum dots disposed between the first electrode and the second electrodes, and a first interfacial layer disposed at the interface between a surface of the layer comprising quantum dots and a first layer in the device. In certain embodiments, a second interfacial layer is optionally further disposed on the surface of the layer comprising quantum dots opposite to the first interfacial layer. In certain embodiments, a device comprises a light-emitting device. Other light emitting devices and methods are disclosed.

This application is a continuation of U.S. patent application Ser. No.13/441,394 filed 6 Apr. 2012, which is a continuation of commonly ownedInternational Application No. PCT/US2010/051867 filed 7 Oct. 2010, whichwas published in the English language as PCT Publication No. WO2011/044391 A1 on 14 Apr. 2011, which International Application claimspriority to U.S. Application No. 61/249,588 filed 7 Oct. 2009. Each ofthe foregoing is hereby incorporated herein by reference in itsentirety.

International Application No. PCT/US2010/051867 is also acontinuation-in-part of U.S. application Ser. No. 12/896,856 filed 2Oct. 2010, which is a continuation of International Application No.PCT/US2009/002123 filed 3 Apr. 2009, which claims priority to U.S.Application No. 61/042,154 filed 3 Apr. 2008.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under AdvancedTechnology Program Award No. 70NANB7H7056 awarded by NIST and withGovernment support under Contract No. 2004*H838109*000 awarded by theCentral Intelligence Agency. The United States has certain rights in theinvention.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to the technical field of devicesincluding quantum dots.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention there is provideda device including a first electrode and a second electrode, a layercomprising quantum dots disposed between the first electrode and thesecond electrode, and a first interfacial layer disposed at theinterface between a surface of the layer comprising quantum dots and afirst layer in the device.

Preferably, the first interfacial layer is a distinct layer.

In certain embodiments, the first layer can comprise a material capableof transporting charge, for example, a material capable of transportingholes, a material capable of transporting electrons, a material capableof transporting and injecting electrons, etc.

In certain embodiments, the first layer can comprise a material capableof injecting charge, for example, a material capable of injecting holes,a material capable of injecting electrons, etc.

In certain embodiments, the first layer can comprise a metal or otherconductive material.

In certain embodiments, the first layer can comprise one or moreseparate layers.

In certain embodiments, the first layer can comprise one or moreinorganic materials.

In certain embodiments, the first layer can comprise one or more organicmaterials.

In certain embodiments, a second interfacial layer can be disposed atthe interface of the surface of the layer comprising quantum dotsopposite the first interfacial layer and a second layer in the device.Preferably, the second interfacial layer is a distinct layer.

In certain embodiments, the second layer can comprise a material capableof transporting charge, for example, a material capable of transportingholes, a material capable of transporting electrons, a material capableof transporting and injecting electrons, etc.

In certain embodiments, the second layer can comprise a material capableof injecting charge, for example, a material capable of injecting holes,a material capable of injecting electrons, etc.

In certain embodiments, the second layer can comprise a metal or otherconductive material.

In certain embodiments, the second layer can comprise one or moreseparate layers.

In certain embodiments, the second layer can comprise one or moreinorganic material.

In certain embodiments, the second layer can comprise one or moreorganic materials.

An interfacial layer can comprise one or more separate layers.

An interfacial layer can comprise one or more inorganic materials.

An interfacial layer can comprise one or more organic materials.

An interfacial can fill voids that may exist between quantum dots.

An interfacial layer preferably can protect quantum dots from chargequenching sites in another device layer.

An interfacial layer can preferably protect quantum dots from chargequenching sites in a contiguous device layer.

An interfacial layer can comprise an adhesion promoting moiety. Examplesof compounds including such moieties include, but are not limited to,surfactants.

An interfacial layer included in the devices and/or light emittingdevices taught herein can comprise a surfactant (including but notlimited to silicon-containing coupling agents). Examples include, butare not limited to, 1,4-bis(trimethoxysilylethyl)benzene,diphenyldiethoxysilane, other silane coupling agents including a phenylgroup and/or a hydrolyzable alkoxy functional group. Other examplesinclude, but are not limited to, surfactants or compounds that includefunctional groups such as amines, thiols, phosphonic acids, carboxylicacids, and other functional groups of the type typically included inligands for quantum dots.

One example of a technique for forming an interfacial layer comprising asurfactant is a spin-coating technique. In certain of such embodiments,for example, the surfactant can be diluted with a volatilizable solvent(typically organic (e.g., hexane, etc.), spun onto the surface to becoated, and dried (e.g., baking in air at 100-150° C.). In embodimentsthat may include an interfacial layer comprising a surfactant, it may bedesirable to apply the the surfactant in the thinnest possible thicknessto minimize interference of electrial conductivity between the electrontransport layer and the quantum dots.

An interfacial layer can comprise a metal oxide. Examples of metaloxides include metal oxides described elsewhere herein. In certain ofsuch embodiments, an interfacial layer comprising a metal oxidecomprises a separate layer added to the device (as opposed to a metaloxide formed by oxidation of a material in another layer of the device).

In certain embodiments, an interfacial layer can comprise a metal oxideincluding an alkali metal or alkaline earth metal dopant (such aslithium, sodium, potassium, cesium, magnesium, calcium, barium, etc.).In certain of such embodiments, the dopant level is about 10% or less,about 5% or less, about 2% or less, about 1% or less. In certainembodiments, a doped metal oxide can be formed by a sol-gel techniquewherein the dopant is added by including a salt of the desired alkalimetal or alkaline earth metal in the metal oxide precursor sol-gelmixture in an amount based on the desired dopant level for the dopedmetal oxide material.

An interfacial layer can comprise an organic small molecule material(e.g., but not limited to, OXD-7, LG101, S-2NPB, and other smallmolecule materials typically used in organic light emitting devicesand/or quantum dot light emitting devices that include small moleculecharge transport materials).

In certain embodiments, an interfacial layer can comprise organic smallmolecules that chemically stabilize the surface of the contiguous firstor second layer, as the case may be.

In certain embodiments, an interfacial layer can comprise organic smallmolecules having a dipole moment that modifies the work function of thecontiguous first or second layer, as the case may be.

Interfacial layers comprising organic small molecules can optionally beformed by phase separation of a mixture including quantum dots and theorganic small molecule material.

Interfacial layers can be formed by a number of different techniques,including, but not limited to, spincasting, atomic layer deposition(ALD), molecular layer deposition (MLD), physical vapor deposition(e.g., evaporation, sputtering, electron beam evaporation), chemicalvapor deposition (CVD), plasma-enhanced chemical vapor deposition(PECVD), contact printing, inkjet printing, self-assembly techniques,etc. Other suitable techniques can also be used.

In certain embodiments, an interfacial layer comprises a solutionprocessible material. Solution processible materials are desirable andcan be preferred for use in fabricating devices.

An interfacial layer preferably comprises a material non-quenching toquantum dot emission.

An interfacial layer can comprise a material that is non-crystallizing.For example, crystallizing of the material in the interfacial layerduring device fabrication and device operation can be undesirable.

An interfacial layer can comprise a material with a glass transitiontemperature (Tg) greater than 150° C.

An interfacial layer can comprise a spiro compound.

An interfacial layer can comprise a conformal wide band gap material,such as, for example, but not limited to, metal oxides (e.g., aluminumoxide, hafnium oxide, etc.)

An interfacial layer can comprise non-light-emitting nanoparticleshaving a bandgap that is the same as or similar to the bandgap ofquantum dots included in the active or emissive layer of the devicecomprising quantum dots. In certain embodiments, non-light-emittingnanoparticles can comprise non-emissive quantum dots.

An interfacial layer can comprise non-light-emitting nanoparticleshaving a bandgap that is higher than the bandgap of quantum dotsincluded in the active or emissive layer of the device comprisingquantum dots. In certain embodiments, non-light-emitting nanoparticlescan comprise non-emissive quantum dots.

An interfacial layer included on the electron-injecting side of a deviceand/or light emitting device taught herein can preferably comprisenon-light-emitting nanoparticles having a similar LUMO levels to quantumdots included in the active or emissive device layer including quantumdots.

An interfacial layer included on the electron-injecting side of a deviceand/or light emitting device taught herein can preferably comprisenon-light-emitting nanoparticles having a similar HOMO levels to quantumdots included in the active or emissive device layer including quantumdots.

An interfacial layer can preferably comprise non-light-emittingsemiconductor nanoparticles that have been chemically treated to givethem intrinsic semiconductor properties.

An interfacial can comprise non-light-emitting semiconductornanoparticles that have been chemically treated to give them n-type(electron transporting) semiconductor properties.

An interfacial layer can comprise non-light-emitting semiconductornanoparticles that have been chemically treated to give them p-type(hole transporting) semiconductor properties.

An interfacial layer can comprise non-light-emitting nanoparticles thathave been chemically treated to include a chemical linker capable ofattaching to the emissive layer.

An interfacial layer can comprise an inorganic material that chemicallystabilizes the surface of the first layer.

An interfacial layer included can comprise a bipolar transport material.

An interfacial layer can comprise an organometallic complex.

An interfacial layer can comprise a material with a weak dipole moment.

An interfacial layer can comprise a material with a strong dipolemoment.

An interfacial layer can comprise a material with no dipole moment.

An interfacial layer can be attached to the layer comprising quantumdots and/or the first layer by an interfacial layer comprising linkermolecules.

In a device including emissive quantum dots, an interfacial layerpreferably has a thickness effective to reduce quenching of quantum dotemission due to interaction of the quantum dots with the first layer.

An interfacial layer preferably further has a thickness less than athickness that would reduce charge transfer or tunneling between thelayer comprising quantum dots and the first layer.

In certain embodiments, an interfacial layer can have a thicknessranging from a monolayer thickness to about 5 nm. In certainembodiments, an interfacial layer can have a thickness ranging from amonolayer thickness to about 10 nm. In certain embodiments, aninterfacial layer can have a thickness ranging from a monolayerthickness to about 15 nm. In certain embodiments, an interfacial layercan have a thickness ranging from a monolayer thickness to about 20 nm.In certain embodiments, an interfacial layer can have a thicknessranging from a monolayer thickness to about 25 nm.

In certain embodiments, a monolayer thickness can have a thickness ofapproximately the diameter of a molecule included in the interfaciallayer. In certain embodiments, the interfacial layer has a thickness upto about 10 monolayers. In certain embodiments, the interfacial layerhas a thickness up to about 5 monolayers. In certain embodiments, theinterfacial layer has a thickness up to about 3 monolayers. In certainembodiments, the interfacial layer has a thickness of about 2monolayers. In certain embodiments, the interfacial layer has athickness of about 1 monolayer.

Other thicknesses outside the above examples may also be determined tobe useful or desirable.

The first interfacial layer can comprise an interfacial layer describedherein.

The second interfacial layer, if included, can comprise an interfaciallayer described herein.

In certain embodiments, the device comprises a light emitting device andthe layer comprising quantum dots is an emissive layer.

In certain embodiments, the device comprises a light emitting device inaccordance with embodiments of the invention taught herein.

In accordance with another aspect of the present invention, there isprovided a light emitting device including a first electrode and asecond electrode, an emissive layer comprising quantum dots disposedbetween the first and second electrodes, a first layer comprising amaterial capable of transporting charge disposed between the firstelectrode and the emissive layer, and a first interfacial layer disposedbetween the emissive layer and the first layer comprising a materialcapable of transporting charge.

Preferably the first interfacial layer is a distinct layer.

The first interfacial layer can comprise an interfacial layer describedherein.

In certain embodiments, the first layer can comprise one or moreseparate layers.

In certain embodiments, the first layer can comprise a material capableof injecting and transporting charge.

A material capable of transporting charge (e.g., holes or electrons) cancomprise an organic material.

Mixtures or blends of two or more organic materials can also be used.

A material capable of transporting charge (e.g., holes or electrons)preferably comprises an inorganic material.

Examples of such inorganic materials include, but are not limited to,metal chalcogenides. Examples of metal chalcogenides include, but arenot limited to, metal oxides and metal sulfides.

One example of a preferred inorganic material capable of transportingcharge comprises zinc oxide.

Mixtures or blends of two or more inorganic materials can also be used.

In certain embodiments, a material capable of transporting charge or amaterial capable of injecting and transporting charge can comprise astratified structure including two or more horizontal zones or layers.

A light emitting device in accordance with the invention can furtherinclude a second layer comprising a material capable of transportingcharge between the emissive layer and the second electrode.

The second layer comprising a material capable of transporting charge(e.g., holes or electrons) can comprise a material capable oftransporting charge described herein.

In certain embodiments, the second layer can comprise one or moreseparate layers.

A light emitting device in accordance with the invention can furtherinclude a second interfacial layer between the emissive layer and thesecond layer comprising a material capable of transporting charge in thedevice. Preferably, the second interfacial layer is a distinct layer.

The second interfacial layer, if included, can comprise an interfaciallayer described herein.

In certain embodiments, the first electrode comprises a cathode and thesecond electrode comprises an anode.

In certain embodiments, the first electrode comprises an anode and thesecond electrode comprises a cathode.

In certain embodiments, the first layer comprises a material capable oftransporting electrons.

In certain embodiments, the material capable of transporting electronsis further capable of injecting electrons.

In certain embodiments, the first layer comprises a material capable oftransporting holes.

In certain embodiments, the device further includes a second layercomprising a material capable of transporting charge between theemissive layer and the second electrode.

In certain embodiments, the second layer comprises a material capable oftransporting holes. In certain of such embodiments, the first layer cancomprise a material capable of transporting electrons. In certainembodiments, the material capable of transporting electrons is furthercapable of injecting electrons.

In certain embodiments, the second layer comprises a material capable oftransporting electrons. In certain of such embodiments, the first layercan comprise a material capable of transporting holes.

In certain embodiments, the second layer comprises a material capable oftransporting and injecting electrode electrons. In certain of suchembodiments, the first layer can comprise a material capable oftransporting holes.

In certain embodiments, one or more additional layers (e.g., but notlimited to, charge injection, charge blocking, etc.) can be included inthe device.

In certain embodiments, a light emitting device includes a firstelectrode and a second electrode, and an emissive layer comprisingquantum dots provided between the electrodes, a first layer comprisingmaterial capable of transporting and injecting electrons providedbetween the first electrode and the emissive layer, a first interfaciallayer between the emissive layer and layer comprising material capableof transporting and injecting electrons, a second layer comprisingmaterial capable of transporting holes provided between the emissivelayer and the second electrode, and a layer comprising a hole-injectionmaterial provided between the second electrode and the layer comprisingmaterial capable of transporting holes. In certain preferredembodiments, the first electrode comprises a cathode and the secondelectrode comprises an anode.

In certain embodiments, the material capable of transporting electronscomprises an inorganic material.

In certain embodiments, the material capable of transporting electronscomprises an organic material.

In certain embodiments, the material capable of transporting electronsis further capable of injection electrons.

In certain embodiments, the material capable of transporting andinjecting electrons comprises an inorganic material. In certain of suchembodiments, such inorganic material is doped with a species to enhanceelectron transport characteristics of the inorganic material.

In certain embodiments, a material capable of transporting electronscomprises an inorganic semiconductor material.

In certain embodiments, a material capable of transporting and injectingelectrons comprises an inorganic semiconductor material.

In certain embodiments, a material capable of transporting electronscomprises a metal chalcogenide. In certain embodiments, a materialcapable of transporting electrons comprises a metal sulfide. In certainpreferred embodiments, a material capable of transporting electronscomprises a metal oxide.

In certain embodiments, a material capable of transporting and injectingelectrons comprises a metal chalcogenide. In certain embodiments, amaterial capable of transporting and injecting electrons comprises ametal sulfide. In certain preferred embodiments, a material capable oftransporting and injecting electrons comprises a metal oxide.

In certain embodiments, an inorganic material comprises an inorganicsemiconductor material. Nonlimiting examples include metal chalcogenides(e.g., metal oxides, metal sulfides, etc.). In certain embodiments, aninorganic material comprises titanium dioxide. In certain more preferredembodiments, an inorganic material comprises zinc oxide. In certainembodiments, an inorganic material comprises a mixture of two or moreinorganic materials. In certain preferred embodiments, an inorganicmaterial comprises a mixture of zinc oxide and titanium oxide.

In certain embodiments, a device includes the following layers formed inthe following sequential order: the first electrode (preferablycomprising a cathode), the first layer comprising a material capable oftransporting electrons, a first interfacial layer, the emissive layercomprising quantum dots, the second layer comprising a material capableof transporting holes comprising, the layer comprising a hole injectionmaterial, and the second electrode (preferably comprising an anode).

In certain embodiments, a material capable of transporting electronscomprises an inorganic material. In certain of such embodiments, theinorganic material comprises an inorganic semiconductor material.

In certain embodiments, the layer comprising a material capable oftransporting electrons (and preferably further capable of injectingelectrons) can comprise a stratified structure including two or morehorizontal zones having different conductivities. In certainembodiments, the stratified structure includes a first zone, on a sideof the structure closer to the first electrode (preferably comprising acathode), comprising an n-type doped material with electron injectingcharacteristics, and a second zone, on the side of the structure closerto the emissive layer, comprising an intrinsic or lightly doped materialwith electron transport characteristics. In certain embodiments, forexample, the first zone can comprise n-type doped zinc oxide and thesecond zone can comprise intrinsic zinc oxide or n-type doped zinc oxidewith a lower n-type dopant concentration that that of the zinc oxide inthe first zone. In certain embodiments, for example, the stratifiedstructure can include a first zone, on a side of the structure closer tothe first electrode (preferably comprising a cathode), comprising ann-type doped material with electron injecting characteristics, a thirdzone, on a side of the structure closer to the emissive layer,comprising an intrinsic material with hole blocking characteristics, anda second zone, between the first and third zones, comprising anintrinsic or lightly doped material with electron transportcharacteristics. In certain embodiments, for example, a layer comprisinga material capable of transporting and injecting electrons can comprisea first layer, closer to the first electrode (preferably comprising acathode), comprising a material capable of injecting electrons and asecond layer, closer to the emissive layer, comprising a materialcapable of transporting electrons. In certain embodiments, for example,a layer comprising a material capable of transporting and injectingelectrons can comprise a first layer, closer to the cathode; a secondlayer, closer to the emissive layer, comprising a material capable ofblocking holes; and a third layer between the first and second layers,comprising a material capable of transporting electrons.

In certain embodiments, the material capable of transporting holes cancomprise an organic material.

In certain embodiments, the device can further include a secondinterfacial layer at the interface between the emissive layer and thesecond layer. Preferably, the second interfacial layer is a distinctlayer.

In certain embodiments, a hole injection material can comprise amaterial capable of transporting holes that is p-type doped.

In certain embodiments, the absolute value of the difference betweenE_(LUMO) of the quantum dots and the Work function of the Cathode isless than 0.5 eV. In certain embodiments, the absolute value of thedifference between E_(LUMO) of the quantum dots and the Work function ofthe Cathode is less than 0.3 eV. In certain embodiments, the absolutevalue of the difference between E_(LUMO) of the quantum dots and theWork function of the Cathode is less than 0.2 eV.

In certain embodiments, the absolute value of the difference betweenE_(LUMO) of the quantum dots and E_(conduction band edge) of thematerial capable of transporting & injecting electrons is less than 0.5eV. In certain embodiments, the absolute value of the difference betweenE_(LUMO) of the quantum dots and E_(conduction band edge) of materialcapable of transporting & injecting electrons is less than 0.3 eV. Incertain embodiments, the absolute value of the difference betweenE_(LUMO) of the quantum dots and E_(conduction band edge) of materialcapable of transporting & injecting electrons is less than 0.2 eV.

In certain embodiments, the absolute value of the difference betweenE_(HOMO) of the quantum dots and the E_(VALENCE band edge) of thematerial capable of transporting and injecting electrons is greater thanabout 1 eV. In certain embodiments, the absolute value of the differencebetween E_(HOMO) of the quantum dots and the E_(VALENCE band edge) ofthe material capable of transporting and injecting electrons is greaterthan about 0.5 eV. In certain embodiments, the absolute value of thedifference between E_(HOMO) of the quantum dots and theE_(VALENCE band edge) of the material capable of transporting andinjecting electrons is greater than about 0.3 eV.

In certain embodiments, an anode comprising a material with <5 eV workfunction can be used, thereby avoiding the need to utilize preciousmetals such as gold, etc.

In certain embodiments, the device can have an initial turn-on voltagethat is not greater than 1240/λ, wherein λ represents the wavelength(nm) of light emitted by the emissive layer.

In certain embodiments, light emission from the light emissive materialoccurs at a bias across the device that is less than the electron-Voltof the bandgap of the quantum dots in the emissive layer.

An example of a preferred embodiment of a light emitting device inaccordance with the present invention comprises a pair of electrodes, alayer comprising a light emissive material comprising quantum dotsprovided between the electrodes, a first layer comprising a materialcapable of transporting electrons provided between the emissive layerand one of the electrodes, and a first interfacial layer disposed at theinterface between the emissive layer and the first layer comprising amaterial capable of transporting electrons, wherein the first layercomprising the material capable of transporting electrons comprising aninorganic material comprises a stratified structure including two ormore horizontal zones having different conductivities. The inorganicmaterial included in different zones of the stratified structure can bedoped or undoped forms of the same or different materials.

In certain embodiments, the electron and hole populations are balancedat the emissive layer of the device.

In certain embodiments, the material capable of transporting electronscomprises an inorganic material.

In certain embodiments, the material capable of transporting electronscomprises a material that is further capable of injection electrons. Incertain embodiments, such material comprises an inorganic material.

In certain embodiments, the inorganic material comprises an inorganicsemiconductor material.

In certain preferred embodiments, the inorganic material comprises ametal chalcogenide. In certain embodiments, the inorganic materialcomprises a metal sulfide. In certain preferred embodiments, theinorganic material comprises a metal oxide. In certain embodiments, theinorganic material comprises titanium dioxide.

In certain more preferred embodiments, the inorganic material compriseszinc oxide. In certain embodiments, the zinc oxide is surface treatedwith an oxidizing agent to render the surface proximate to the emissivelayer intrinsic.

In certain embodiments, the inorganic material can comprise a mixture oftwo or more inorganic materials.

In certain embodiments, the layer comprising a stratified structure astaught herein can serve as a layer capable of transporting and injectingelectrons. In certain embodiments, a zone in a layer comprising astratified structure as taught herein can have a predeterminedconductivity so as to serve as a layer capable of transportingelectrons, a layer capable of injecting electrons, and/or a layercapable of blocking holes. In certain embodiments, a zone can comprise adistinct layer.

In certain embodiments, one or more additional layers taught herein canbe included in the device.

In certain embodiments, a second interfacial layer can be included inthe device on the surface of the emissive layer opposite the firstinterfacial layer. Preferably, the second interfacial layer is adistinct layer.

The first interfacial layer and the second interfacial layer (ifincluded) can comprise an interfacial layer described herein.

In certain embodiments of the light emitting devices taught herein, thedevice has an initial turn-on voltage that is not greater than 1240/λ,wherein λ represents the wavelength (nm) of light emitted by theemissive layer.

In certain embodiments of a light emitting device in accordance with thepresent invention, light emission from the light emissive materialoccurs at a bias voltage across the device that is less than the energyin electron-Volts of the bandgap of the emissive material.

In certain embodiments, the light emitting device includes an emissivematerial comprising quantum dots. In certain embodiments, other wellknown light emissive materials can further be used or included in thedevice. In certain embodiments, additional layers can also be included.

In accordance with another aspect of the invention, there are provideddisplays and other products including the above-described light-emittingdevice.

In accordance with another aspect of the present invention, there isprovided a method for preparing a device described herein, the methodcomprising:

forming a first electrode on a substrate;

forming a first layer thereover;

applying a first interfacial layer thereover;

applying a layer comprising quantum dots thereover; and

forming a second electrode thereover.

In certain embodiments, the method further comprises forming a secondinterfacial layer over the surface of the layer comprising quantum dotsopposite the first interfacial layer. Preferably, the second interfaciallayer is a distinct layer.

Examples of electrodes and interfacial layers are described herein.

In certain embodiments, other layers described herein can be included inthe device.

In certain embodiment, the method further includes encapsulating thedevice.

In certain embodiments, the first electrode comprises a cathode and thesecond electrode comprises an anode.

In certain embodiments, the first electrode comprises an anode and thesecond electrode comprises a cathode.

In certain embodiments, the device comprises a light emitting device andthe method comprises:

forming a first electrode on a substrate;

forming a first layer comprising a material capable of transportingcharge thereover;

applying a first interfacial layer thereover;

applying an emissive layer comprising quantum dots thereover; and

forming second electrode thereover.

In certain embodiments, the method further comprises forming a secondinterfacial layer over the surface of the emissive layer opposite thefirst interfacial layer. Preferably, the second interfacial layer is adistinct layer.

Examples of interfacial layers are described herein.

In certain embodiments, other layers described herein can be included inthe device.

In certain embodiments, the method further comprises encapsulating thelight emitting device.

In certain embodiments, the first electrode comprises a cathode and thesecond electrode comprises an anode.

In certain embodiments, the first electrode comprises an anode and thesecond electrode comprises a cathode.

Quantum dots that can be included in a device or method taught hereincan comprise quantum dots including a core comprising a first materialand a shell disposed over at least a portion of, and preferablysubstantially all, of the outer surface of the core, the shellcomprising a second material. (A quantum dot including a core and shellis also described herein as having a core/shell structure.) Optionally,more than one shell can be included on the core. The first material canpreferably comprise an inorganic semiconductor material and the secondmaterial can preferably comprise an inorganic semiconductor material.

Preferably quantum dots comprise inorganic semiconductor nanocrystals.Such inorganic semiconductor nanocrystals preferably comprise acore/shell structure. In certain preferred embodiments, quantum dotscomprise colloidally grown inorganic semiconductor nanocrystals.

Quantum dots typically can include a ligand attached to an outer surfacethereof. In certain embodiments, two or more chemically distinct ligandscan be attached to an outer surface of at least a portion of the quantumdots.

A layer including quantum dots that can be included in a device ormethod taught herein can include two or more different types of quantumdots, wherein each type is selected to emit light having a predeterminedwavelength. In certain embodiments, quantum dot types can be differentbased on, for example, factors such composition, structure and/or sizeof the quantum dot.

Quantum dots can be selected to emit at any predetermined wavelengthacross the electromagnetic spectrum.

An emissive layer can include different types of quantum dots that haveemissions at different wavelengths.

In certain embodiments, quantum dots can be capable of emitting visiblelight.

In certain embodiments, quantum dots can be capable of emitting infraredlight.

As used herein, the terms “inorganic material” and “organic material”may be further defined by a functional descriptor, depending on thedesired function being addressed. In certain embodiments, the samematerial can address more than one function.

In certain embodiments, it may be desirable to have differentconductivities which can be accomplished, for example, by changing thecarrier mobility and/or charge density of a material in a zone and/orlayer.

In certain embodiments including a stratified structure, horizontalzones are preferably parallel to the electrodes.

Other aspects and embodiments of the invention relate to materials andmethods that are useful in making the above described devices.

The foregoing, and other aspects described herein, all constituteembodiments of the present invention.

It should be appreciated by those persons having ordinary skill in theart(s) to which the present invention relates that any of the featuresdescribed herein in respect of any particular aspect and/or embodimentof the present invention can be combined with one or more of any of theother features of any other aspects and/or embodiments of the presentinvention described herein, with modifications as appropriate to ensurecompatibility of the combinations. Such combinations are considered tobe part of the present invention contemplated by this disclosure.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the invention as claimed. Other embodimentswill be apparent to those skilled in the art from consideration of thedescription and drawings, from the claims, and from practice of theinvention disclosed herein.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is schematic drawing depicting an example of an embodiment of alight-emitting device structure in accordance with the invention.

The attached FIGURE is a simplified representation presented forpurposes of illustration only; actual structures may differ in numerousrespects, including, e.g., relative scale, etc.

For a better understanding to the present invention, together with otheradvantages and capabilities thereof, reference is made to the followingdisclosure and appended claims in connection with the above-describeddrawings.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 provides a schematic representation of an example of thearchitecture of a light-emitting device according to one embodiment ofthe present invention. Referring to FIG. 1, the light-emitting device 10includes (from top to bottom) a second electrode (e.g., an anode) 1, asecond layer comprising a material capable of transporting charge (e.g.,a material capable of transporting holes, which is also referred toherein as a “hole transport material”) 2, an emissive layer includingquantum dots 3, a first interfacial layer 4, a first layer comprising amaterial capable of transporting charge (e.g., a material capable oftransporting electrons, a material capable of transporting and injectingelectrons, such materials also being referred to herein as an “electrontransport material”) 5, a first electrode (e.g., a cathode) 6, and asubstrate (not shown). In certain embodiments, a second interfaciallayer is optionally further included between the emissive layer and thesecond layer. If included, a second interfacial layer is preferably adistinct layer.

In certain preferred embodiments, the electron transport materialcomprises an inorganic material.

In certain embodiments, the anode is proximate to and injects holes intothe hole transport material while the cathode is proximate to andinjects electrons into the electron transport material. The injectedholes and injected electrons combine to form an exciton on the quantumdot and emit light. In certain embodiments, a hole injection layer isfurther included between the anode and the hole transport layer.

In certain preferred embodiments, an electron transport material is alsocapable of injecting electrons.

The substrate (not shown) can be opaque or transparent. A transparentsubstrate can be used, for example, in the manufacture of a transparentlight emitting device. See, for example, Bulovic, V. et al., Nature1996, 380, 29; and Gu, G. et al., Appl. Phys. Lett. 1996, 68, 2606-2608,each of which is incorporated by reference in its entirety. Thesubstrate can be rigid or flexible. The substrate can be plastic, metal,semiconductor wafer, or glass. The substrate can be a substrate commonlyused in the art. Preferably the substrate has a smooth surface. Asubstrate surface free of defects is particularly desirable.

The cathode 6 can be formed on the substrate (not shown). In certainembodiments, a cathode can comprise, ITO, aluminum, silver, gold, etc.The cathode preferably comprises a material with a work function chosenwith regard to the quantum dots included in the device. In certainembodiments, the absolute value of the difference between E_(LUMO) ofthe quantum dots and the work function of the cathode is less than about0.5 eV. In certain embodiments the absolute value of the differencebetween E_(LUMO) of the quantum dots and the work function of thecathode is less than about 0.3 eV, and preferably less than about 0.2eV. E_(LUMO) of the quantum dots represents the energy level of thelowest unoccupied molecular orbital (LUMO) of the quantum dot. Forexample, a cathode comprising indium tin oxide (ITO) can be preferredfor use with an emissive material including quantum dots comprising aCdSe core/CdZnSe shell.

Substrates including patterned ITO are commercially available and can beused in making a device according to the present invention.

The layer comprising a material capable of transporting electrons 5preferably comprises an inorganic material. Preferably the materialcapable of transporting electrons also is capable of injectingelectrons. In certain embodiments, the inorganic material included inthe layer capable or transporting and injection electrons comprises aninorganic semiconductor material. Preferred inorganic semiconductormaterials include those having a band gap that is greater than theemission energy of the emissive material. In certain embodiments, theabsolute value of the difference between E_(LUMO) of the quantum dotsand E_(conduction band edge) of material capable of transporting andinjecting electrons, is less than about 0.5 eV. In certain embodiments,the absolute value of the difference between E_(LUMO) of the quantumdots and E_(conduction band edge) of the material capable oftransporting and injecting electrons, is less than about 0.3 eV, andpreferably less than about 0.2 eV E_(LUMO) of the quantum dotsrepresents the energy level of the lowest unoccupied molecular orbital(LUMO) of the quantum dots; E_(of the conduction band edge) of thematerial capable of transporting and injecting electrons represents theenergy level of the conduction band edge of the material capable oftransporting and injecting electrons.

Examples of inorganic semiconductor materials include a metalchalcogenide, a metal pnictide, or elemental semiconductor, such as ametal oxide, a metal sulfide, a metal selenide, a metal telluride, ametal nitride, a metal phosphide, a metal arsenide, or metal arsenide.For example, an inorganic semiconductor material can include, withoutlimitation, zinc oxide, a titanium oxide, a niobium oxide, an indium tinoxide, copper oxide, nickel oxide, vanadium oxide, chromium oxide,indium oxide, tin oxide, gallium oxide, manganese oxide, iron oxide,cobalt oxide, aluminum oxide, thallium oxide, silicon oxide, germaniumoxide, lead oxide, zirconium oxide, molybdenum oxide, hafnium oxide,tantalum oxide, tungsten oxide, cadmium oxide, iridium oxide, rhodiumoxide, ruthenium oxide, osmium oxide, zinc sulfide, zinc selenide, zinctelluride, cadmium sulfide, cadmium selenide, cadmium telluride, mercurysulfide, mercury selenide, mercury telluride, silicon carbide, diamond(carbon), silicon, germanium, aluminum nitride, aluminum phosphide,aluminum arsenide, aluminum antimonide, gallium nitride, galliumphosphide, gallium arsenide, gallium antimonide, indium nitride, indiumphosphide, indium arsenide, indium antimonide, thallium nitride,thallium phosphide, thallium arsenide, thallium antimonide, leadsulfide, lead selenide, lead telluride, iron sulfide, indium selenide,indium sulfide, indium telluride, gallium sulfide, gallium selenide,gallium telluride, tin selenide, tin telluride, tin sulfide, magnesiumsulfide, magnesium selenide, magnesium telluride, barium titanate,barium zirconate, zirconium silicate, yttria, silicon nitride, and amixture of two or more thereof. In certain embodiments, the inorganicsemiconductor material can include a dopant.

In certain preferred embodiments, an electron transport material caninclude an n-type dopant.

An example of a preferred inorganic semiconductor material for inclusionin an electron transport material of a device in accordance with theinvention is zinc oxide. In certain embodiments, zinc oxide can be mixedor blended with one or more other inorganic materials, e.g., inorganicsemiconductor materials, such as titanium oxide.

As mentioned above, in certain preferred embodiments, a layer comprisinga material capable of transporting and injecting electrons can comprisezinc oxide. Such zinc oxide can be prepared, for example, by a sol-gelprocess. In certain embodiments, the zinc oxide can be chemicallymodified. Examples of chemical modification include treatment withhydrogen peroxide.

In other preferred embodiments, a layer comprising a material capable oftransporting and injecting electrons can comprise a mixture includingzinc oxide and titanium oxide.

The electron transport material is preferably included in the device asa layer. In certain embodiments, the layer has a thickness in a rangefrom about 10 nm to 500 nm.

Electron transport materials comprising an inorganic semiconductormaterial can be deposited at a low temperature, for example, by a knownmethod, such as a vacuum vapor deposition method, an ion-plating method,sputtering, inkjet printing, sol-gel, etc. For example, sputtering istypically performed by applying a high voltage across a low-pressure gas(for example, argon) to create a plasma of electrons and gas ions in ahigh-energy state. Energized plasma ions strike a target of the desiredcoating material, causing atoms from that target to be ejected withenough energy to travel to, and bond with, the substrate.

In certain embodiments, the layer comprising a material capable oftransporting and injecting electrons can comprise a stratified structurecomprising an inorganic material, wherein the stratified structureincludes two or more horizontal zones having different conductivities.For example, in certain embodiments, the layer can include a first zoneat the upper portion of the layer (nearer the emissive layer) comprisingan intrinsic or slightly n-type doped inorganic material (e.g.,sputtered intrinsic or slightly n-type doped zinc oxide) with electrontransporting characteristics, and a second zone at the lower portion ofthe layer (more remote from the emissive layer) comprising inorganicmaterial that has a higher concentration of n-type doping than thematerial in the first zone (e.g., sputtered n-type doped ZnO) withelectron injection characteristics.

In another example, in certain embodiments, the layer can include threehorizontal zones, e.g., a first zone at the upper portion of the layer(nearest the emissive layer) comprising an intrinsic inorganic material(e.g., sputtered intrinsic zinc oxide) which can be hole blocking; asecond zone (between the first zone and the third zone) comprising anintrinsic or slightly n-type doped inorganic material (e.g., sputteredintrinsic or slightly n-type doped zinc oxide or another metal oxide)which can be electron transporting; and a third zone at the lowestportion of the layer (most remote from the emissive layer) comprisinginorganic material that has a higher concentration of n-type doping thanthe material in the second zone (e.g., sputtered n-type doped ZnO oranother metal oxide) which can be hole injecting.

In certain embodiments, the inorganic material included in thestratified structure comprises an inorganic semiconductor material. Incertain preferred embodiments, the inorganic material comprises a metalchalcogenide. In certain embodiments, the inorganic material comprises ametal sulfide. In certain preferred embodiments, the inorganic materialcomprises a metal oxide. In certain embodiments, the inorganic materialcomprises titanium dioxide. In certain more preferred embodiments, theinorganic material comprises zinc oxide. In certain embodiments, theinorganic material can comprise a mixture of two or more inorganicmaterials. Other inorganic materials taught herein for inclusion in alayer comprising a material capable of transporting and injectionelectrons can also be included in a stratified structure.

Additional information concerning inorganic materials that may be usefulfor inclusion in an electron transport layer is disclosed inInternational Application No. PCT/US2006/005184, filed 15 Feb. 2006, for“Light Emitting Device Including Semiconductor Nanocrystals, whichpublished as WO 2006/088877 on 26 Aug. 2006, the disclosure of which ishereby incorporated herein by reference in its entirety.

The surface of the device on which an inorganic semiconductor materialis to be formed can be cooled or heated for temperature control duringthe growth process. The temperature can affect the crystallinity of thedeposited material as well as how it interacts with the surface it isbeing deposited upon. The deposited material can be polycrystalline oramorphous. The deposited material can have crystalline domains with asize in the range of 10 Angstroms to 1 micrometer. If doped, the dopingconcentration can be controlled by, for example, varying the gas, ormixture of gases, with a sputtering plasma technique. The nature andextent of doping can influence the conductivity of the deposited film,as well as its ability to optically quench neighboring excitons.

In certain embodiments, a material capable of transporting electrons cancomprise an organic material. Information related to fabrication oforganic charge transport layers that may be helpful are disclosed inU.S. patent application Ser. No. 11/253,612 for “Method And System ForTransferring A Patterned Material”, filed 21 Oct. 2005, and Ser. No.11/253,595 for “Light Emitting Device Including SemiconductorNanocrystals”, filed 21 Oct. 2005, each of which is hereby incorporatedherein by reference in its entirety. Other organic electron transportmaterials can be readily identified by one of ordinary skill in therelevant art.

In the example shown in FIG. 1, the first interfacial layer is disposedbetween the first layer 5 and the emissive layer 3. The inclusion of thefirst interfacial layer between the first layer and emissive layer canpreferably reduce the photoluminescent quenching of quantum dots whilenot impeding charge flow.

An interfacial layer can comprise an inorganic material.

An interfacial layer can comprise one or more inorganic materials.

An interfacial layer can comprise an organic material.

An interfacial layer can comprise one or more organic materials.

An interfacial layer can comprise one or more separate layers.

In certain embodiments, the interfacial layer can fill voids that mayexist between quantum dots.

In certain preferred embodiments, the interfacial layer can protectquantum dots from charge quenching sites in another device layer.Quenching sites can include, for example, but are not limited to,degraded organic molecules included, e.g., in a device layer, highconductivity materials that may be included in charge injection devicelayers, dangling bonds that may occur in, e.g., inorganic chargetransport materials (e.g., metal oxides).

An interfacial layer included in the devices and/or light emittingdevices taught herein can comprise an adhesion promoting moiety.Examples of compounds including such moieties include, but are notlimited to, surfactants.

In certain embodiments, an interfacial layer comprises a surfactant(including but not limited to silicon-containing coupling agents).Examples include, but are not limited to,1,4-bis(trimethoxysilylethyl)benzene, diphenyldiethoxysilane, othersilane coupling agents including a phenyl group and/or a hydrolyzablealkoxy functional group. Other examples include, but are not limited to,surfactants or compounds that include functional groups such as amines,thiols, phosphonic acids, carboxylic acids, and other functional groupsof the type typically included in ligands for quantum dots.

In certain embodiments in which an interfacial layer comprising asurfactant is included between an electron transport material and alayer including quantum dots, the surfactant is applied with thethinnest possible thickness to minimize interference of electrialconductivity between the electron transport layer and the quantum dots.

One example of a technique for forming an interfacial layer comprising asurfactant is a spin-coating technique. In certain of such embodiments,for example, the surfactant can be diluted with a volatilizable solvent(typically organic (e.g., hexane, etc.), spun onto the surface to becoated, and dried (e.g., baking in air at 100-150° C.).

In certain embodiments, an interfacial layer comprises a metal oxide.Examples of metal oxides include wide band gap metal oxide materials(e.g., aluminum oxide, hafnium oxide, etc.) and other metal oxidesdescribed elsewhere herein.

In certain embodiments, an interfacial layer comprises a metal oxideincluding an alkali metal or alkaline earth metal dopant (such aslithium, sodium, potassium, cesium, magnesium, calcium, barium, etc.).In certain of such embodiments, the dopant level is about 10% or less,about 5% or less, about 2% or less, about 1% or less. In certainembodiments, a doped metal oxide can be formed by a sol-gel techniquewherein the dopant is added by including a salt of the desired alkalimetal or alkaline earth metal in the metal oxide precursor sol-gelmixture in an amount based on the desired dopant level for the dopedmetal oxide material.

In certain embodiments, an interfacial layer comprises an organic smallmolecule material (e.g., but not limited to, OXD-7, LG101, S-2NPB, andother small molecule materials typically used in organic light emittingdevices and/or quantum dot light emitting devices that include smallmolecule charge transport materials).

An interfacial layer included in the devices and/or light emittingdevices taught herein can comprise organic small molecules thatchemically stabilize the surface of the contiguous first or secondlayer, as the case may be.

An interfacial layer included in the devices and/or light emittingdevices taught herein can comprise organic small molecules having adipole moment that modifies the work function of the contiguous first orsecond layer, as the case may be.

Interfacial layers can optionally be formed by phase separation of amixture including quantum dots and organic small molecule material.

Interfacial layers can be formed by a number of different techniques,including, but not limited to, spincasting, atomic layer deposition(ALD), physical vapor deposition (e.g., evaporation, sputtering,electron beam evaporation), molecular layer deposition (MLD), chemicalvapor deposition (CVD), plasma-enhanced chemical vapor deposition(PECVD), contact printing, inkjet printing, self-assembly techniques,etc. Such techniques are known. Other suitable techniques can also beused.

In certain embodiments, the interfacial layer comprises a solutionprocessible material. Solution processible materials are desirable andcan be preferred for use in fabricating devices.

In certain preferred embodiments, an interfacial layer comprises amaterial non-quenching to quantum dot emission.

In certain embodiments, an interfacial layer comprises a material thatis non-crystallizing. For example, crystallizing of the material in theinterfacial layer during device fabrication and device operation can beundesirable.

In certain embodiments, an interfacial layer comprises material with aglass transition temperature (Tg) greater than 150° C.

In certain embodiments, an interfacial layer comprises a spiro compound.

An interfacial layer included in the devices and/or light emittingdevices taught herein can comprise a conformal wide band gap material,such as, for example, but not limited to, metal oxides (e.g., aluminumoxide, hafnium oxide, etc.)

An interfacial layer included in light emitting devices taught hereincan preferably comprise non-light-emitting nanoparticles having abandgap that is the same or similar to the bandgap of quantum dotsincluded in the emissive layer. Examples of such nanoparticles include,but are not limited to, nanoparticles comprising CdSe, CdS, ZnSe, CdTe,or ZnTe.

An interfacial layer included in light emitting devices taught hereincan preferably comprise non-light-emitting nanoparticles having abandgap that is higher than the bandgap of quantum dots included in theemissive layer. Examples of such nanoparticles include, but are notlimited to, nanoparticles comprising ZnO, TiO₂, ZnS, CuAlO₂, WO₃, ZrO₂,or associated alloys.

An interfacial layer included on the electron-injecting side of a deviceand/or light emitting device taught herein can preferably comprisenon-light-emitting nanoparticles having a similar LUMO levels to quantumdots included in the active or emissive device layer including quantumdots.

An interfacial layer included on the electron-injecting side of a deviceand/or light emitting device taught herein can preferably comprisenon-light-emitting nanoparticles having a similar HOMO levels to quantumdots included in the active or emissive device layer including quantumdots.

An interfacial layer included in the devices and/or light emittingdevices taught herein can preferably comprise non-light-emittingsemiconductor nanoparticles that have been chemically treated to givethem intrinsic semiconductor properties.

In embodiments in which an interfacial layer comprises nanoparticles(e.g., semiconductor nanoparticles, quantum dots, semiconductornanocrystals, etc.), such nanoparticles can further include ligandgroups attached thereto which are chemically or physicallydistinguishable from those that may be attached to quantum dots includedin the device layer comprising quantum dots. By way of non-limitingexample, if the quantum dots included in the device layer have longchain ligands attached thereto, the nanoparticles included in aninterfacial layer can include short chain ligands attached thereto.Selection of ligands based on this teaching is within the skill of theperson of ordinary skill in the relevant art.

An interfacial layer included in the devices and/or light emittingdevices taught herein can comprise non-light-emitting semiconductornanoparticles that have been chemically treated to give them n-type(electron transporting) semiconductor properties.

An interfacial layer included in the devices and/or light emittingdevices taught herein can comprise non-light-emitting semiconductornanoparticles that have been chemically treated to give them p-type(hole transporting) semiconductor properties.

Examples of chemical treatments include, but are not limited to, in situligand exchange. Chemical treatments could alternatively or additionallybe performed at the synthesis stage. Examples of synthesis-stageprocessing include, but are not limited to, solution-phase ligandexchange or incorporation of dopant during nanocrystal growth.

An interfacial layer included in the devices and/or light emittingdevices taught herein can comprise non-light-emitting nanoparticles thathave been chemically treated to include a chemical linker capable ofattaching to the emissive layer. Examples of chemical linkers include,but are not limited to, compounds including functional groups such asamines, thiols, phosphonic acids, carboxylic acids, and other functionalgroups of the type typically included in ligands for quantum dots.

An interfacial layer included in the devices and/or light emittingdevices taught herein can comprise an inorganic material that chemicallystabilizes the surface of the first layer.

An interfacial layer included in the devices and/or light emittingdevices taught herein can comprise a bipolar transport material.

An interfacial layer included can comprise an organometallic complex.

An interfacial layer included can comprise a material with a weak dipolemoment.

An interfacial layer included can comprise a material with a strongdipole moment.

An interfacial layer included can comprise a material with no dipolemoment.

An interfacial layer can be attached to the layer comprising quantumdots and/or the first layer by an interfacial layer comprising linkermolecules.

An interfacial layer is preferably a distinct layer (e.g., a layercomprising a material that is physically or chemically distinguishablefrom a contiguous device layer or a separate layer as opposed to a dopedregion of contiguous layer).

In certain embodiments, the interfacial layer has a thickness effectiveto prevent charge quenching of quantum dots due to interaction of thequantum dots in the device layer comprising quantum dots with anotherlayer or material which would be contiguous thereto if the interfaciallayer were not disposed therebetween. Preferably, the thickness of theinterfacial layer is also selected to not prevent charge transfer ortunneling between the emissive layer and such layer.

In a light emitting device, the interfacial layer preferably has athickness effective to prevent charge quenching of quantum dot emissiondue to interaction of the quantum dots in the emissive with anotherlayer or material which would be contiguous thereto if the interfaciallayer were not disposed therebetween. Preferably, the thickness of theinterfacial layer is also selected to not prevent charge transfer ortunneling between the emissive layer and such layer.

In certain embodiments, an interfacial layer can have a thicknessranging from a monolayer thickness to about 5 nm. In certainembodiments, an interfacial layer can have a thickness ranging from amonolayer thickness to about 10 nm. In certain embodiments, aninterfacial layer can have a thickness ranging from a monolayerthickness to about 15 nm. In certain embodiments, an interfacial layercan have a thickness ranging from a monolayer thickness to about 20 nm.In certain embodiments, an interfacial layer can have a thicknessranging from a monolayer thickness to about 25 nm. In certainembodiments, a monolayer thickness can have a thickness of approximatelythe diameter of a molecule included in the layer. Other thicknessesoutside the above examples may also be determined to be useful ordesirable.

In certain embodiments, an interfacial layer can promote betterelectrical interface between the emissive layer and the layer of thedevice on the other side of the interfacial layer.

The emissive material 4 includes quantum dots. In certain embodiments,the quantum dots comprise an inorganic semiconductor material. Incertain preferred embodiments, the quantum dots comprise crystallineinorganic semiconductor material (also referred to as semiconductornanocrystals). Examples of preferred inorganic semiconductor materialsinclude, but are not limited to, Group II-VI compound semiconductornanocrystals, such as CdS, CdSe, ZnS, ZnSe, ZnTe, HgS, HgSe, HgTe, andother binary, ternary, and quaternary II-VI compositions; Group III-Vcompound semiconductor nanocrystals, such as GaP, GaAs, InP and InAs;PbS; PbSe; PbTe, and other binary, ternary, and quaternary III-Vcompositions. Other non-limiting examples of inorganic semiconductormaterials include Group II-V compounds, Group III-VI compounds, GroupIV-VI compounds, Group I-III-VI compounds, Group II-IV-VI compounds,Group II-IV-V compounds, Group IV elements, an alloy including any ofthe foregoing, and/or a mixture including any of the foregoing. Further,materials for the quantum dot light-emitting layer may be core-shellstructured nanocrystals (for example, CdSe/ZnS, CdS/ZnSe, InP/ZnS, etc.)wherein the core is composed of a semiconductor nanocrystal (e.g. CdSe,CdS, etc.) and the shell is composed of a crystalline inorganicsemiconductor material (e.g., ZnS, ZnSe, etc.).

Quantum dots can also have various shapes, including, but not limitedto, sphere, rod, disk, other shapes, and mixtures of various shapedparticles.

An emissive material can comprise one or more different quantum dots.The differences can be based, for example, on different composition,different size, different structure, or other distinguishingcharacteristic or property.

The color of the light output of a light-emitting device can becontrolled by the selection of the composition, structure, and size ofthe quantum dots included in a light-emitting device as the emissivematerial.

The emissive material is preferably included in the device as a layer.In certain embodiments, the emissive layer can comprise one or morelayers of the same or different emissive material(s). In certainembodiments, the emissive layer can have a thickness in a range fromabout 1 nm to about 20 nm. In certain embodiments, the emissive layercan have a thickness in a range from about 1 nm to about 10 nm. Incertain embodiments, the emissive layer can have a thickness in a rangefrom about 3 nm to about 6 about nm. In certain embodiments, theemissive layer can have a thickness of about 4 nm. A thickness of 4 nmcan be preferred in a device including an electron transport materialincluding a metal oxide. Other thicknesses outside the above examplesmay also be determined to be useful or desirable.

Preferably, the quantum dots include one or more ligands attached to thesurface thereof. In certain embodiments, a ligand can include an alkyl(e.g., C₁-C₂₀) species. In certain embodiments, an alkyl species can bestraight-chain, branched, or cyclic. In certain embodiments, an alkylspecies can be substituted or unsubstituted. In certain embodiments, analkyl species can include a hetero-atom in the chain or cyclic species.In certain embodiments, a ligand can include an aromatic species. Incertain embodiments, an aromatic species can be substituted orunsubstituted. In certain embodiments, an aromatic species can include ahetero-atom. Additional information concerning ligands is providedherein and in various of the below-listed documents which areincorporated herein by reference.

By controlling the structure, shape and size of quantum dots duringpreparation, energy levels over a very broad range of wavelengths can beobtained while the properties of the bulky materials are varied. Quantumdots (including but not limited to semiconductor nanocrystals) can beprepared by known techniques. Preferably they are prepared by a wetchemistry technique wherein a precursor material is added to acoordinating or non-coordinating solvent (typically organic) andnanocrystals are grown so as to have an intended size. According to thewet chemistry technique, when a coordinating solvent is used, as thequantum dots are grown, the organic solvent is naturally coordinated tothe surface of the quantum dots, acting as a dispersant. Accordingly,the organic solvent allows the quantum dots to grow to thenanometer-scale level. The wet chemistry technique has an advantage inthat quantum dots of a variety of sizes can be uniformly prepared byappropriately controlling the concentration of precursors used, the kindof organic solvents, and preparation temperature and time, etc.

A coordinating solvent can help control the growth of quantum dots. Thecoordinating solvent is a compound having a donor lone pair that, forexample, has a lone electron pair available to coordinate to a surfaceof the growing quantum dots. Solvent coordination can stabilize thegrowing quantum dot. Examples of coordinating solvents include alkylphosphines, alkyl phosphine oxides, alkyl phosphonic acids, or alkylphosphinic acids, however, other coordinating solvents, such aspyridines, furans, and amines may also be suitable for quantum dotproduction. Additional examples of suitable coordinating solventsinclude pyridine, tri-n-octyl phosphine (TOP), tri-n-octyl phosphineoxide (TOPO) and trishydroxylpropylphosphine (tHPP), tributylphosphine,tri(dodecyl)phosphine, dibutyl-phosphite, tributyl phosphite,trioctadecyl phosphite, trilauryl phosphite, tris(tridecyl) phosphite,triisodecyl phosphite, bis(2-ethylhexyl)phosphate, tris(tridecyl)phosphate, hexadecylamine, oleylamine, octadecylamine,bis(2-ethylhexyl)amine, octylamine, dioctylamine, trioctylamine,dodecylamine/laurylamine, didodecylamine tridodecylamine,hexadecylamine, dioctadecylamine, trioctadecylamine, phenylphosphonicacid, hexylphosphonic acid, tetradecylphosphonic acid, octylphosphonicacid, octadecylphosphonic acid, propylenediphosphonic acid,phenylphosphonic acid, aminohexylphosphonic acid, dioctyl ether,diphenyl ether, methyl myristate, octyl octanoate, and hexyl octanoate.In certain embodiments, technical grade TOPO can be used.

Quantum dots can alternatively be prepared with use of non-coordinatingsolvent(s).

Size distribution during the growth stage of the reaction can beestimated by monitoring the absorption or emission line widths of theparticles. Modification of the reaction temperature in response tochanges in the absorption spectrum of the particles allows themaintenance of a sharp particle size distribution during growth.Reactants can be added to the nucleation solution during crystal growthto grow larger crystals. For example, for CdSe and CdTe, by stoppinggrowth at a particular semiconductor nanocrystal average diameter andchoosing the proper composition of the semiconducting material, theemission spectra of the semiconductor nanocrystals can be tunedcontinuously over the wavelength range of 300 nm to 5 microns, or from400 nm to 800 nm.

The particle size distribution of quantum dots can be further refined bysize selective precipitation with a poor solvent for the quantum dots,such as methanol/butanol as described in U.S. Pat. No. 6,322,901. Forexample, semiconductor nanocrystals can be dispersed in a solution of10% butanol in hexane. Methanol can be added dropwise to this stirringsolution until opalescence persists. Separation of supernatant andflocculate by centrifugation produces a precipitate enriched with thelargest crystallites in the sample. This procedure can be repeated untilno further sharpening of the optical absorption spectrum is noted.Size-selective precipitation can be carried out in a variety ofsolvent/nonsolvent pairs, including pyridine/hexane andchloroform/methanol. The size-selected quantum dot population preferablyhas no more than a 15% rms deviation from mean diameter, more preferably10% rms deviation or less, and most preferably 5% rms deviation or less.

In certain embodiments, quantum dots preferably have ligands attachedthereto.

In certain embodiment, the ligands can be derived from the coordinatingsolvent used during the growth process.

In certain embodiments, the surface can be modified by repeated exposureto an excess of a competing coordinating group to form an overlayer.

For example, a dispersion of the capped semiconductor nanocrystal can betreated with a coordinating organic compound, such as pyridine, toproduce crystallites which disperse readily in pyridine, methanol, andaromatics but no longer disperse in aliphatic solvents. Such a surfaceexchange process can be carried out with any compound capable ofcoordinating to or bonding with the outer surface of the semiconductornanocrystal, including, for example, phosphines, thiols, amines andphosphates. The semiconductor nanocrystal can be exposed to short chainpolymers which exhibit an affinity for the surface and which terminatein a moiety having an affinity for a liquid medium in which thesemiconductor nanocrystal is suspended or dispersed. Such affinityimproves the stability of the suspension and discourages flocculation ofthe semiconductor nanocrystal.

More specifically, the coordinating ligand can have the formula:(Y—)_(k-n)—(X)-(-L)_(n)wherein k is 2, 3 4, or 5, and n is 1, 2, 3, 4 or 5 such that k-n is notless than zero; X is O, O—S, O—Se, O—N, O—P, O—As, S, S═O, SO₂, Se,Se═O, N, N═O, P, P═O, C═O As, or As═O; each of Y and L, independently,is H, OH, aryl, heteroaryl, or a straight or branched C2-18 hydrocarbonchain optionally containing at least one double bond, at least onetriple bond, or at least one double bond and one triple bond. Thehydrocarbon chain can be optionally substituted with one or more C1-4alkyl, C2-4 alkenyl, C2-4 alkynyl, C1-4 alkoxy, hydroxyl, halo, amino,nitro, cyano, C3-5 cycloalkyl, 3-5 membered heterocycloalkyl, aryl,heteroaryl, C1-4 alkylcarbonyloxy, C1-4 alkyloxycarbonyl, C1-4alkylcarbonyl, or formyl. The hydrocarbon chain can also be optionallyinterrupted by —O—, —S—, —N(Ra)—, —N(Ra)—C(O)—O—, —O—C(O)—N(Ra)—,—N(Ra)—C(O)—N(Rb)—, —O—C(O)—O—, —P(Ra)—, or —P(O)(Ra)—. Each of Ra andRb, independently, is hydrogen, alkyl, alkenyl, alkynyl, alkoxy,hydroxylalkyl, hydroxyl, or haloalkyl. An aryl group is a substituted orunsubstituted cyclic aromatic group. Examples include phenyl, benzyl,naphthyl, tolyl, anthracyl, nitrophenyl, or halophenyl. A heteroarylgroup is an aryl group with one or more heteroatoms in the ring, forinstance furyl, pyridyl, pyrrolyl, phenanthryl.

A suitable coordinating ligand can be purchased commercially or preparedby ordinary synthetic organic techniques, for example, as described inJ. March, Advanced Organic Chemistry.

Other ligands are described in U.S. patent application Ser. No.10/641,292 for “Stabilized Semiconductor Nanocrystals”, filed 15 Aug.2003, which issued on 9 Jan. 2007 as U.S. Pat. No. 7,160,613, which ishereby incorporated herein by reference in its entirety.

Other examples of ligands include benzylphosphonic acid,benzylphosphonic acid including at least one substituent group on thering of the benzyl group, a conjugate base of such acids, and mixturesincluding one or more of the foregoing. In certain embodiments, a ligandcomprises 4-hydroxybenzylphosphonic acid, a conjugate base of the acid,or a mixture of the foregoing. In certain embodiments, a ligandcomprises 3,5-di-tert-butyl-4-hydroxybenzylphosphonic acid, a conjugatebase of the acid, or a mixture of the foregoing.

Additional examples of ligands that may be useful with the presentinvention are described in International Application No.PCT/US2008/010651, filed 12 Sep. 2008, of Breen, et al., for“Functionalized Nanoparticles And Method” and International ApplicationNo. PCT/US2009/004345, filed 28 Jul. 2009 of Breen et al., for“Nanoparticle Including Multi-Functional Ligand And Method”, each of theforegoing being hereby incorporated herein by reference.

The emission from a quantum dot capable of emitting light (e.g., asemiconductor nanocrystal) can be a narrow Gaussian emission band thatcan be tuned through the complete wavelength range of the ultraviolet,visible, or infra-red regions of the spectrum by varying the size of thequantum dot, the composition of the quantum dot, or both. For example, asemiconductor nanocrystal comprising CdSe can be tuned in the visibleregion; a semiconductor nanocrystal comprising InAs can be tuned in theinfra-red region. The narrow size distribution of a population ofquantum dots capable of emitting light (e.g., semiconductornanocrystals) can result in emission of light in a narrow spectralrange. The population can be monodisperse preferably exhibits less thana 15% rms (root-mean-square) deviation in diameter of such quantum dots,more preferably less than 10%, most preferably less than 5%. Spectralemissions in a narrow range of no greater than about 75 nm, no greaterthan about 60 nm, no greater than about 40 nm, and no greater than about30 nm full width at half max (FWHM) for such quantum dots that emit inthe visible can be observed. IR-emitting quantum dots can have a FWHM ofno greater than 150 nm, or no greater than 100 nm. Expressed in terms ofthe energy of the emission, the emission can have a FWHM of no greaterthan 0.05 eV, or no greater than 0.03 eV. The breadth of the emissiondecreases as the dispersity of the light-emitting quantum dot diametersdecreases.

For example, semiconductor nanocrystals can have high emission quantumefficiencies such as greater than 10%, 20%, 30%, 40%, 50%, 60%, 70%,80%, or 90%.

The narrow FWHM of semiconductor nanocrystals can result in saturatedcolor emission. The broadly tunable, saturated color emission over theentire visible spectrum of a single material system is unmatched by anyclass of organic chromophores (see, for example, Dabbousi et al., J.Phys. Chem. 101, 9463 (1997), which is incorporated by reference in itsentirety). A monodisperse population of semiconductor nanocrystals willemit light spanning a narrow range of wavelengths. A pattern includingmore than one size of semiconductor nanocrystal can emit light in morethan one narrow range of wavelengths. The color of emitted lightperceived by a viewer can be controlled by selecting appropriatecombinations of semiconductor nanocrystal sizes and materials. Thedegeneracy of the band edge energy levels of semiconductor nanocrystalsfacilitates capture and radiative recombination of all possibleexcitons.

Transmission electron microscopy (TEM) can provide information about thesize, shape, and distribution of the semiconductor nanocrystalpopulation. Powder X-ray diffraction (XRD) patterns can provide the mostcomplete information regarding the type and quality of the crystalstructure of the semiconductor nanocrystals. Estimates of size are alsopossible since particle diameter is inversely related, via the X-raycoherence length, to the peak width. For example, the diameter of thesemiconductor nanocrystal can be measured directly by transmissionelectron microscopy or estimated from X-ray diffraction data using, forexample, the Scherrer equation. It also can be estimated from the UV/Visabsorption spectrum.

An emissive material can be deposited by spin-casting, screen-printing,inkjet printing, gravure printing, roll coating, drop-casting,Langmuir-Blodgett techniques, contact printing or other techniques knownor readily identified by one skilled in the relevant art.

In certain preferred embodiments, an emissive layer comprising quantumdots (which quantum dots may further include ligands attached thereto)included in a light emitting device (or an active layer comprisingquantum dots (which quantum dots may further include ligands attachedthereto) included in a non-light emitting device) described hereinincludes quantum dots that are not dispersed in a host matrix.

In certain preferred embodiments, after the emissive material isdeposited, it is exposed to small molecules and/or light prior toforming another device layer thereover. Examples of small moleculesinclude a molecule with a molecular weight of less than 100 a.m.u.,e.g., water. Small polar molecules can be preferred. A small moleculecan be in the form of a gas, a liquid dispersed in carrier gas (e.g., amist, vapor, spray, etc.), a liquid, and/or a mixture thereof. Mixturesincluding small molecules having different compositions can also beused. A small molecule can include a lone electron pair. Such exposureto small molecules and/or light can be carried out in air or in theabsence or substantial absence of oxygen. Exposure to small moleculesand/or light can be carried out at a temperature in a range from about20° to about 80° C. When carried out in light, the light can include apeak emission wavelength that can excite at least a portion of thequantum dots. For example, light can include a peak emission wavelengthin a range from about 365 nm to about 480 nm. Light can be provided by alight source with peak wavelength at a desired wavelength. Light fluxcan be in a range from about 10 to about 100 mW/cm². See also, forexample, U.S. Application Nos. 61/377,242 of Peter T. Kazlas, et al.,entitled “Device Including Quantum Dots”, filed 26 Aug. 2010, and61/377,148 of Peter T. Kazlas, et al., entitled “Quantum Dot LightEmitting Device”, filed 26 Aug. 2010, each of the foregoing being herebyincorporated herein by reference in its entirety.

Examples of hole transport materials include organic material andinorganic materials. An example of an organic material that can beincluded in a hole transport layer includes an organic chromophore. Theorganic chromophore can include a phenyl amine, such as, for example,N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine(TPD). Other hole transport layer can include(N,N′-bis(3-methylphenyl)-N,N′-bis(phenyl)-spiro (spiro-TPD),4-4′-N,N′-dicarbazolyl-biphenyl (CBP),4,4-.bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPD), etc., apolyaniline, a polypyrrole, a poly(phenylene vinylene), copperphthalocyanine, an aromatic tertiary amine or polynuclear aromatictertiary amine, a 4,4′-bis(p-carbazolyl)-1,1′-biphenyl compound,N,N,N′,N′-tetraarylbenzidine, poly(3,4-ethylenedioxythiophene)(PEDOT)/polystyrene para-sulfonate (PSS) derivatives,poly-N-vinylcarbazole derivatives, polyphenylenevinylene derivatives,polyparaphenylene derivatives, polymethacrylate derivatives,poly(9,9-octylfluorene) derivatives, poly(spiro-fluorene) derivatives,N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB),tris(3-methylphenylphenylamino)-triphenylamine (m-MTDATA), andpoly(9,9′-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine (TFB), andspiro-NPB.

In certain preferred embodiments, a hole transport layer comprises anorganic small molecule material, a polymer, a spiro-compound (e.g.,spiro-NPB), etc.

In certain embodiments of the inventions described herein, a holetransport layer can comprise an inorganic material. Examples ofinorganic materials include, for example, inorganic semiconductormaterials capable of transporting holes. The inorganic material can beamorphous or polycrystalline Examples of such inorganic materials andother information related to fabrication of inorganic hole transportmaterials that may be helpful are disclosed in International ApplicationNo. PCT/US2006/005184, filed 15 Feb. 2006, for “Light Emitting DeviceIncluding Semiconductor Nanocrystals, which published as WO 2006/088877on 26 Aug. 2006, the disclosure of which is hereby incorporated hereinby reference in its entirety.

Hole transport materials comprising, for example, an inorganic materialsuch as an inorganic semiconductor material, can be deposited at a lowtemperature, for example, by a known method, such as a vacuum vapordeposition method, an ion-plating method, sputtering, inkjet printing,sol-gel, etc.

Organic hole transport materials may be deposited by known methods suchas a vacuum vapor deposition method, a sputtering method, a dip-coatingmethod, a spin-coating method, a casting method, a bar-coating method, aroll-coating method, and other film deposition methods. Preferably,organic layers are deposited under ultra-high vacuum (e.g., <10⁸ torr),high vacuum (e.g., from about 10⁻⁸ torr to about 10⁻⁵ torr), or lowvacuum conditions (e.g., from about 10⁻⁵ torr to about 10⁻³ torr).

Hole transport materials comprising organic materials and otherinformation related to fabrication of organic charge transport layersthat may be helpful are disclosed in U.S. patent application Ser. No.11/253,612 for “Method And System For Transferring A PatternedMaterial”, filed 21 Oct. 2005, and Ser. No. 11/253,595 for “LightEmitting Device Including Semiconductor Nanocrystals”, filed 21 Oct.2005, each of which is hereby incorporated herein by reference in itsentirety.

The hole transport material is preferably included in the device as alayer. In certain embodiments, the layer can have a thickness in a rangefrom about 10 nm to about 500 nm.

Device 10 can further include a hole-injection material. Thehole-injection material may comprise a separate hole injection materialor may comprise an upper portion of the hole transport layer that hasbeen doped, preferably p-type doped. The hole-injection material can beinorganic or organic. Examples of organic hole injection materialsinclude, but are not limited to, LG-101 (see, for example, paragraph(0024) of EP 1 843 411 A1) and other HIL materials available from LGChem, LTD. Other organic hole injection materials can be used. Examplesof p-type dopants include, but are not limited to, stable, acceptor-typeorganic molecular material, which can lead to an increased holeconductivity in the doped layer, in comparison with a non-doped layer.In certain embodiments, a dopant comprising an organic molecularmaterial can have a high molecular mass, such as, for example, at least300 amu. Examples of dopants include, without limitation, F₄-TCNQ,FeCl₃, etc. Examples of doped organic materials for use as a holeinjection material include, but are not limited to, an evaporated holetransport material comprising, e.g., 4,4′,4″-tris(diphenylamino)triphenylamine (TDATA) that is doped withtetrafluoro-tetracyano-quinodimethane (F₄-TCNQ); p-doped phthalocyanine(e.g., zinc-phthalocyanine (ZnPc) doped with F₄-TCNQ (at, for instance,a molar doping ratio of approximately 1:30);N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′biphenyl-4,4″diamine (alpha-NPD)doped with F₄-TCNQ. See J. Blochwitz, et al., “Interface ElectronicStructure Of Organic Semiconductors With Controlled Doping Levels”,Organic Electronics 2 (2001) 97-104; R. Schmechel, 48, InternationalesWissenschaftliches Kolloquium, Technische Universtaat Ilmenau, 22-25Sep. 2003; C. Chan et al., “Contact Potential Difference Measurements OfDoped Organic Molecular Thin Films”, J. Vac. Sci. Technol. A 22(4),July/August 2004. The disclosures of the foregoing papers are herebyincorporated herein by reference in their entireties. See also, Examplesof p-type doped inorganic hole transport materials are described in U.S.Patent Application No. 60/653,094 entitled “Light Emitting DeviceIncluding Semiconductor Nanocrystals, filed 16 Feb. 2005, which ishereby incorporated herein by reference in its entirety. Examples ofp-type doped organic hole transport materials are described in U.S.Provisional Patent Application No. 60/795,420 of Beatty et al, for“Device Including Semiconductor Nanocrystals And A Layer Including ADoped Organic Material And Methods”, filed 27 Apr. 2006, which is herebyincorporated herein by reference in its entirety.

As shown in FIG. 1, anode 1 may comprise an electrically conductivemetal or its oxide that can easily inject holes. Examples include, butare not limited to, ITO, aluminum, aluminum-doped zinc oxide (AZO),silver, gold, etc. Other suitable anode materials are known and can bereadily ascertained by the skilled artisan. The anode material can bedeposited using any suitable technique. In certain embodiments, theanode can be patterned.

In certain embodiments, the light-emitting device may be fabricated bysequentially forming the first electrode (e.g., a cathode) 6, the firstlayer (e.g., comprising an electron transport material) 5, the firstinterfacial layer 4, the emissive material 3, the second layer (e.g.,comprising a hole transport material) 2, and the second electrode (e.g.,an anode) 1. In certain embodiments, a second interfacial layer isincluded between the emissive layer 3 and the second layer 2.

In embodiments of devices described herein that include a first andsecond interfacial layer, the emissive or active device layer includingquantum dots preferably is sufficiently thick to prevent shortingbetween the two interfacial layers.

In certain embodiments, the surface of a charge transport layercomprising a metal oxide can be treated with ozone to promote adhesionof the layer to be disposed thereon. Other adhesion promotion techniquescan be used.

In certain embodiments, the electrode (e.g., anode or cathode) materialsand other materials are selected based on the light transparencycharacteristics thereof so that a device can be prepared that emitslight from the top surface thereof. A top emitting device can beadvantageous for constructing an active matrix device (e.g., a display).In certain embodiments, the electrode (e.g., anode or cathode) materialsand other materials are selected based on light transparencycharacteristics thereof so that a device can be prepared that emitslight from the bottom surface thereof.

As mentioned above, the device can further include a substrate (notshown in the FIGURE). Examples of substrate materials include, withoutlimitation, glass, plastic, insulated metal foil.

In certain embodiments, a device can further include a passivation orother protective layer that can be used to protect the device from theenvironment. For example, a protective glass layer can be included toencapsulate the device. Optionally a desiccant or other moistureabsorptive material can be included in the device before it is sealed,e.g., with an epoxy, such as a UV curable epoxy. Other desiccants ormoisture absorptive materials can be used.

In accordance with another aspect of the present invention, there isprovided a method for preparing a device described herein. In certainembodiments, the method comprises: forming a first layer over a firstelectrode (e.g., a cathode); forming an interfacial layer over the firstlayer, forming a layer comprising quantum dots thereover; and forming asecond electrode (e.g., an anode) thereover. In certain embodiments, themethod can further include forming a second interfacial layer on thelayer comprising quantum dots prior to formation of another devicelayer. For example, in certain embodiments, the method further includesforming a second layer over the emissive layer. In certain of suchembodiments, the method can further include forming a layer comprising ahole injection material over the second layer. Examples of materialsthat can be included in the method include those described herein.

Other information and techniques described herein and incorporated byreference can also be useful in practicing a method in accordance withthe present invention.

In certain embodiments, the method can be used to prepare a lightemitting device including a pair of electrodes, a layer comprising alight emissive material comprising quantum dots provided between theelectrodes, a first layer comprising a material capable of transportingelectrons comprising an inorganic material provided between the emissivelayer and one of the electrodes, wherein the layer comprising thematerial capable of transporting electrons comprising an inorganicmaterial comprises a stratified structure including two or morehorizontal zones having different conductivities, and a firstinterfacial layer between the emissive layer and the first layercomprising the material capable of transporting electrons. The inorganicmaterial included in different zones of the stratified structure can bedoped or undoped forms of the same or different materials.

In certain embodiments, the inorganic material comprises an inorganicsemiconductor material. For example, if a first zone comprises anintrinsic inorganic semiconductor material, a second zone, adjacentthereto, can comprise a doped inorganic semiconductor material; if afirst zone comprises an n-type doped inorganic semiconductor material, asecond zone, adjacent thereto, can comprise a slightly lower n-typedoped or intrinsic inorganic semiconductor material. In certainembodiments, the inorganic semiconductor material that is doped can be adoped form of an intrinsic material included in another zone of thestratified structure. While these examples describe a stratifiedstructure including two zones, a stratified structure can include morethan two zones. The inorganic semiconductor material included indifferent zones of the stratified structure can be doped or undopedforms of the same or different materials.

In certain embodiments, the layer comprising a stratified structure canserve as a layer capable of transporting and injecting electrons. Incertain embodiments, a zone in a layer comprising a stratified structurecan have a predetermined conductivity so as to serve as a layer capableof transporting electrons, a layer capable of injecting electrons,and/or a layer capable of blocking holes. In certain embodiments, a zonecan comprise a distinct layer.

In certain embodiments, the inorganic material comprises a metalchalcogenide. In certain embodiments, the inorganic material comprises ametal sulfide. In certain preferred embodiments, the inorganic materialcomprises a metal oxide. In certain embodiments, the inorganic materialcomprises titanium dioxide. In certain more preferred embodiments, theinorganic material comprises zinc oxide. In certain embodiments, theinorganic material comprises a mixture of two or more inorganicmaterials. Other examples of inorganic semiconductor materials that canbe used include those described elsewhere herein.

In certain embodiments, a layer comprising an inorganic semiconductormaterial that includes a stratified structure as taught herein can serveas a layer capable of transporting electrons, injecting electrons,and/or blocking holes.

Examples of materials useful for the anode and cathode include thosedescribed elsewhere herein.

Quantum dots included in the emissive layer can include those describedelsewhere herein. Examples of interfacial layers include those describedelsewhere herein. Optionally, a second interfacial layer can be includedon the side of the emissive layer opposite the first interfacial layer.Preferably, the first and second interfacial layer are distinct layers.

In certain embodiments, different conductivities can be accomplished,for example, by changing the carrier mobility and/or charge density ofthe material.

In certain embodiments including an inorganic material comprising ametal oxide, for example, conduction properties of layers comprising ametal oxide are highly dependent on the concentration of oxygen in thelayer structure since vacancies are the main mode of carrier conduction.For example, in certain embodiments, to control the oxygen concentrationin sputter deposited layers (e.g., made by magnetron RF sputterdeposition) two properties of the deposition can be altered. The powerof deposition can be varied, increasing and decreasing the amount ofoxygen that is incorporated in the layer. The powers and resultingconductivities are highly dependent on the material and the sputtersystem used. More oxygen can also be incorporated into the layer byadding oxygen to the sputter chamber gas environment which is oftendominated by noble gases like Argon. Both the power and oxygen partialpressure can be used or customized to produce the desired layered metaloxide structure. Lowering the RF power during deposition can increasethe conductivity of the layer, reducing the parasitic resistance of thelayer. To deposit a low conductivity layer, oxygen is incorporated intothe deposition ambient to place a thin insulating surface on the layerformed.

Other information and techniques described herein and incorporated byreference can also be useful with this aspect of the present invention.

In certain embodiments of the present invention, there is provided alight emitting device taught herein, wherein light emission from thelight emissive material occurs at a bias voltage across the device thatis less than the energy in electron-Volts of the bandgap of the emissivematerial. In certain embodiments, the light emitting device includes anemissive material comprising quantum dots.

In certain embodiments of the present invention, there is provided alight emitting device taught herein, wherein the device has an initialturn-on voltage that is not greater than 1240/λ, wherein λ representsthe wavelength (nm) of light emitted by the emissive layer.

A light-emitting device in accordance with the invention can be used tomake a light-emitting device including red-emitting, green-emitting,and/or blue-emitting quantum dots. Other color light-emitting quantumdots can be included, alone or in combination with one or more otherdifferent quantum dots. In certain embodiments, separate layers of oneor more different quantum dots may be desirable. In certain embodiments,a layer can include a mixture of two or more different quantum dots.

Light-emitting devices in accordance with various embodiments of theinvention may be incorporated into a wide variety of consumer products,including flat panel displays, computer monitors, televisions,billboards, lights for interior or exterior illumination and/orsignaling, heads up displays, fully transparent displays, flexibledisplays, laser printers, telephones, cell phones, personal digitalassistants (PDAs), laptop computers, digital cameras, camcorders,viewfinders, micro-displays, vehicles, a large area wall, theater orstadium screen, a sign, lamps and various solid state lighting devices.

In certain embodiments, a device taught herein can comprise aphotodetector device including a layer comprising quantum dots selectedbased upon absorption properties. The layer comprising quantum dots isincluded between a pair of electrodes and an interfacial layer isdisposed on at least one surface of the quantum dot containing layer.When included in a photodetector, quantum dots are engineered to producea predetermined electrical response upon absorption of a particularwavelength, typically in the IR or MIR region of the spectrum. Examplesof photodetector devices including quantum dots (e.g., semiconductornanocrystals) are described in “A Quantum Dot HeterojunctionPhotodetector” by Alexi Cosmos Arango, Submitted to the Department ofElectrical Engineering and Computer Science, in partial fulfillment ofthe requirements for the degree of Masters of Science in ComputerScience and Engineering at the Massachusetts Institute of Technology,February 2005, the disclosure of which is hereby incorporated herein byreference in its entirety.

Other materials, techniques, methods, applications, and information thatmay be useful with the present invention are described in: InternationalApplication No. PCT/US2007/008873, filed Apr. 9, 2007, of Coe-Sullivanet al., for “Composition Including Material, Methods Of DepositingMaterial, Articles Including Same And Systems For Depositing Material”;International Application No. PCT/US2007/003411, filed Feb. 8, 2007, ofBeatty, et al., for “Device Including Semiconductor Nanocrystals And ALayer Including A Doped Organic Material And Methods”; InternationalApplication No. PCT/US2007/008721, filed Apr. 9, 2007, of Cox, et al.,for “Methods Of Depositing Nanomaterial & Methods Of Making A Device”;International Application No. PCT/US2007/24320, filed Nov. 21, 2007, ofClough, et al., for “Nanocrystals Including A Group Ma Element And AGroup Va Element, Method, Composition, Device And Other Products”;International Application No. PCT/US2007/24305, filed Nov. 21, 2007, ofBreen, et al., for “Blue Light Emitting Semiconductor Nanocrystal AndCompositions And Devices Including Same”; International Application No.PCT/US2007/013152, filed Jun. 4, 2007, of Coe-Sullivan, et al., for“Light-Emitting Devices And Displays With Improved Performance”;International Application No. PCT/US2007/24310, filed Nov. 21, 2007, ofKazlas, et al., for “Light-Emitting Devices And Displays With ImprovedPerformance”; International Application No. PCT/US2007/003677, filedFeb. 14, 2007, of Bulovic, et al., for “Solid State Lighting DevicesIncluding Semiconductor Nanocrystals & Methods”, U.S. Patent ApplicationNo. 61/016,227, filed 21 Dec. 2007, of Coe-Sullivan et al., for“Compositions, Optical Component, System Including an Optical Component,and Devices”, U.S. Patent Application No. 60/949,306, filed 12 Jul.2007, of Linton, et al., for “Compositions, Methods For DepositingNanomaterial, Methods For Fabricating A Device, And Methods ForFabricating An Array Of Devices”, U.S. Patent Application No.60/992,598, filed 5 Dec. 2007, and International Application No.PCT/US2009/002123, of Zhou, et al. for “Light Emitting Device IncludingQuantum Dots”, filed 3 Apr. 2009. The disclosures of each of theforegoing listed patent documents are hereby incorporated herein byreference in their entireties.

As used herein, the singular forms “a”, “an” and “the” include pluralunless the context clearly dictates otherwise. Thus, for example,reference to an emissive material includes reference to one or more ofsuch materials.

As used herein, “top” and “bottom” are relative positional terms, basedupon a location from a reference point. More particularly, “top” meansfurthest away from the substrate, while “bottom” means closest to thesubstrate. For example, for a light-emitting device including twoelectrodes, the bottom electrode is the electrode closest to thesubstrate, and is generally the first electrode fabricated; the topelectrode is the electrode that is more remote from the substrate, onthe top side of the light-emitting material. The bottom electrode hastwo surfaces, a bottom surface closest to the substrate, and a topsurface further away from the substrate. Where, e.g., a first layer isdescribed as disposed or deposited “over” a second layer, the firstlayer is disposed further away from substrate. There may be layersbetween the first and second layer, unless it is otherwise specified.For example, a cathode may be described as “disposed over” an anode,even though there are various organic and/or inorganic layers inbetween.

The entire contents of all patent publications and other publicationscited in this disclosure are hereby incorporated herein by reference intheir entirety. Further, when an amount, concentration, or other valueor parameter is given as either a range, preferred range, or a list ofupper preferable values and lower preferable values, this is to beunderstood as specifically disclosing all ranges formed from any pair ofany upper range limit or preferred value and any lower range limit orpreferred value, regardless of whether ranges are separately disclosed.Where a range of numerical values is recited herein, unless otherwisestated, the range is intended to include the endpoints thereof, and allintegers and fractions within the range. It is not intended that thescope of the invention be limited to the specific values recited whendefining a range.

Other embodiments of the present invention will be apparent to thoseskilled in the art from consideration of the present specification andpractice of the present invention disclosed herein. It is intended thatthe present specification and examples be considered as exemplary onlywith a true scope and spirit of the invention being indicated by thefollowing claims and equivalents thereof.

The invention claimed is:
 1. A method for preparing a light emittingdevice including a first electrode and a second electrode, an emissivelayer comprising quantum dots disposed between the first electrode andthe second electrode, a first layer disposed between the first electrodeand the layer comprising quantum dots, and a first interfacial layerdisposed at the interface between a surface of the layer comprisingquantum dots and the first layer, the method comprising: forming thefirst electrode on a substrate; forming the first layer thereover, thefirst layer comprising a first charge transport material comprising aninorganic material comprising a metal chalcogenide; applying the firstinterfacial layer thereover for protecting the quantum dots included inthe emissive layer from charge quenching sites in a contiguous devicelayer, the first interfacial layer being applied as a distinct layerhaving a thickness in a range from a monolayer thickness to about 5 nmand comprising a material that is non-quenching to quantum dotphotoluminescent emission and does not impede charge flow; applying theemissive layer comprising quantum dots thereover; and forming the secondelectrode thereover.
 2. A method in accordance with claim 1 furthercomprising forming a second interfacial layer over the surface of thelayer comprising quantum dots opposite the first interfacial layer.
 3. Amethod in accordance with claim 1 wherein the first interfacial layercomprises a surfactant.
 4. A method in accordance with claim 1 whereinthe first interfacial layer comprises a silicon-containing couplingagent.
 5. A method in accordance with claim 1 wherein the firstinterfacial layer comprises a metal oxide.
 6. A method in accordancewith claim 1 wherein the first interfacial layer comprises an organicsmall molecule material.
 7. A method in accordance with claim 1 whereinthe first interfacial layer comprises a metal oxide including an alkalimetal or alkaline earth metal dopant.
 8. A method in accordance withclaim 1 wherein the first interfacial layer comprises non-light-emittingnanoparticles having a bandgap that is the same or similar to thebandgap of quantum dots included in the emissive layer comprisingquantum dots.
 9. A method in accordance with claim 2 further comprisingforming a second layer between the second interfacial layer and thesecond electrode.
 10. A method in accordance with claim 2 wherein thefirst interfacial layer and/or the second interfacial layer is formed byphase separation of a mixture including quantum dots and a smallmolecule material.
 11. A method in accordance with claim 2 wherein thefirst interfacial layer and/or the second interfacial layer is formed byspincasting.
 12. A method in accordance with claim 2 wherein the firstinterfacial layer and/or the second interfacial layer is formed byatomic layer deposition (ALD).
 13. A method in accordance with claim 2wherein the first interfacial layer and/or the second interfacial layeris formed by molecular layer deposition (MLD).
 14. A method inaccordance with claim 2 wherein the first interfacial layer and/or thesecond interfacial layer is formed by physical vapor deposition.
 15. Amethod in accordance with claim 2 wherein the first interfacial layerand/or the second interfacial layer is formed by chemical vapordeposition (CVD).
 16. A method in accordance with claim 2 wherein thefirst interfacial layer and/or the second interfacial layer is formed byplasma-enhanced chemical vapor deposition (PECVD).
 17. A method inaccordance with claim 2 wherein the first interfacial layer and/or thesecond interfacial layer is formed by contact printing.
 18. A method inaccordance with claim 2 wherein the first interfacial layer and/or thesecond interfacial layer is formed by inkjet printing.
 19. A method inaccordance with claim 2 wherein the first interfacial layer and/or thesecond interfacial layer is formed by self-assembly.
 20. A method inaccordance with claim 9 wherein the first layer comprises a materialcapable injecting and transporting electrons, the second layer comprisesa material capable of transporting holes, and wherein the method furtherincludes forming a hole injection layer over the second layer.